Cooling towers are devices that cool process fluid for power plants, process fluid and cooling water for HVAC, for example. Hot process fluid, usually water, is cooled by passing cold ambient air over the hot fluid. The fluid is then cooled by evaporation and/or direct contact with the air. Pumps deliver the process fluid to the tower and in most cases, the process fluid flows by gravity from an elevation inside the cooling tower downwards to the “cold water” basin (usually at the tower base). As the fluid travels downward through the tower, it passes through various types of media that release the fluid into droplets as the fluid continues its path to the bottom of the tower.
During operation, cooling towers generally produce unwanted noise. There are many sources of this noise. These sources include: mechanical devices such as the fan and fan motor, the water from the spray system and general splashing of water over heat transfer media and into the basin, and the air gusting through the cooling tower. Of the sources described above, the falling water is typically a primary contributor to cooling tower noise, especially at ground level near the cooling tower.
As the noise is generated, it is emitted through the air intake and the air outlet of the cooling tower. This noise generated by the cooling tower can be a deterrent to the utilization of a cooling tower in a given application.
There are two primary types of cooling towers used today, counterflow and crossflow. Counterflow towers have become the industry accepted “standard” for large field-erected cooling towers because they generally cost less and consume less footprint area than a comparable crossflow tower. In a counterflow cooling tower, the water passes over the heat transfer media and is cooled by the air as it moves downward. At the bottom of the heat transfer media, the water simply falls unimpeded into the cold water basin below; splashing into the water contained therein, producing noise. In most counterflow towers, the heat transfer media is raised above the operating water level of the cold water basin in order to allow ambient air to enter the tower. As a result of the above-described orientation, this produces a noticeable increase in droplet momentum thereby increasing the amplitude of the noise as it impacts the water surface of the water basin, transferring the energy from the falling fluid droplet to a sound wave as its decent is abruptly halted.
The typical sound level of the noise associated with an operational cooling tower is around 70 dBA at a horizontal distance of 50 feet from the louvered face of the tower. Due to the aforementioned cooling tower operational noise levels, one in every eight field erected counterflow cooling towers, requires some sort of inlet sound attenuation.
Current methods for attenuating noise include slowing the fan or altering the design of the fan with the implementation of variable speed drives and blocking and/or muffling water noise after it has already been created. Slowing and/or modifying the fan gives the cooling tower different cooling properties, is very expensive, and is not applicable in many applications.
With respect to blocking and/or muffling water noise after it is created, there are two primary methods currently employed in the industry. These methods, however, are difficult to maintain, inhibit tower performance, are extremely costly, and/or require the use of large obtrusive walls that can not always be accommodated at the project site.
The first method involves placing a barrier around the cooling tower. One way to do this is to build a wall around the tower or at least around the source of the noise. However, the utilization of a sound barrier is very limited because it is based on the configuration of the project site and any barrier to sound is also a barrier to the air circulation, reducing the effectiveness of the tower. In addition, space is often a premium on a construction site and the ability to build walls around the cooling structures may not be feasible.
The second method involves insulating the side-walls of the cooling tower to prevent the emission of the noise from the tower on a particular side or sides. The use of insulated walls is not possible on the air-entering wall or walls. The object of the cooling tower is to allow air to circulate and cool the water and restricting the circulation of the air is undesirable because it tends to frustrate a central purpose of the cooling tower. To compensate for closing off one or more faces of the air inlet, the tower height and more specifically the air inlet height must increase. Thus, tower framing and the height of the insulated walls are increased which increase costs. Additionally the pumping head is increased which increases operating costs. Furthermore, the falling water height is increased which often results in more noise emitted on the open faces of the tower.
A variation of insulted walls is the use of baffled attenuators. Baffles are aligned across the face of the air inlet with gaps between the baffles to permit air to enter the tower. One or more rows of these baffles may be employed. When two or more rows of insulated baffles are employed, they may be arranged in a staggered pattern to prevent unimpeded sound wave portions from traveling straight out of the air inlet faces. Unlike solid insulated walls, air is permitted to traverse around the baffles and enter the tower. Although, attempts have been made at aerodynamic shaped baffles, this method suffers from air flow pressure loss around the baffles which requires more power to overcome or the loss of thermal performance. By increasing the air inlet height these disadvantages can be overcome at least in part, but the taller attenuation baffles suffer from the same disadvantages as increasing the height of the insulated walls.
Another method attempts to reduce the noise by affecting the falling water, and involves the utilization of droplet interceptors. Water droplets strike the interceptors before being released to the free water surface below. The current droplet interceptors available on the market are made of thick mesh or wood slats sloped at an angle. The thick mesh is problematic because over time it tends to clog, prohibiting water from passing through and enabling biological organisms to grow, creating water treatment issues. In addition, because the mesh is supported by a thin wire “net”, the mesh will tend to sag over time and fall out of the wire “net”, allowing at least some of the falling water to splash unattenuated.
Unlike the mesh, the wood slat material is relatively inexpensive. However the labor involved in installing the wood slats is more expensive than for the mesh. In addition the wood slats are not very effective as a sound attenuation media, in some instances, the slats may actually contribute to the noise of the tower.
Accordingly, it is desirable to provide a sound attenuation method and apparatus that offers a substantial reduction in noise, is relatively inexpensive, maintenance free and avoids restricting the circulation of air within the cooling tower. It is also desirable to provide a noise attenuation structure that is stable, low cost and resists and corrosion.